In recent years, a multi-stage configuration of an automatic transmission progresses so that the number of frictional engagement elements such as a clutch and a brake is increased in accordance with increase in number of gear shift stages. Further, intervals between gear shift lines in a gear shift map become significantly dense in accordance with the increase in number of gear shift stages, whereby a gear shift will readily occur due to a slight change in driving condition (for instance, a throttle opening degree, etc.). That is, gear shift frequency increases, so that a phenomenon in which a request for changing a target gear shift stage is present even during the gear shift more often occurs.
There is conventionally known a device for controlling an automatic transmission as described below (for example, see Patent Literature 1) relating to control to be conducted in such a case that the request for changing the target gear shift stage during the gear shift is outputted. The device is configured that when the request for changing the target gear shift stage is outputted based on a change in driving condition (for instance, a throttle opening degree, etc.) during a time period from gear shift determination to start of an actual gear shift, namely, to start of an inertia phase, the change of the target gear shift stage is allowed, but after the inertia phase is started, the change of the target gear shift stage is prohibited to complete the gear shift during the gear shift.
However, in the above conventional device for controlling an automatic transmission, after the inertia phase is once started, the change of the target gear shift stage is prohibited, but it is not clearly shown at what time the gear shift control to return the gear shift stage should be initiated after the gear shift control is finished. Therefore, when the gear shift control to return the gear shift stage is executed concurrently with the finish of the gear shift control, a shock or a rotation rise of an engine occurs depending on a state of the frictional engagement element released in the prior gear shift control.
Specifically, when the gear shift control to return the gear shift stage is executed, the frictional engagement element released by the prior gear shift control is controlled to be engaged. Here, in the frictional engagement element released in the prior gear shift control, there is a variation in the released state of a piston at the finish of the gear shift. Therefore, when the gear shift control to return the gear shift stage is executed concurrently with the finish of the prior gear shift control under the condition that an actual piston release amount is smaller than that in an estimated piston released state (i.e., when a piston stroke is not returned to an expected amount thereof), there occurs a rapid engagement to thereby cause an engagement shock upon supplying the precharge oil pressure for reducing a backlash. On the contrary, when the gear shift control to return the gear shift stage is executed concurrently with the finish of the prior gear shift control under the condition that the actual piston release amount is larger than that in the estimated piston released state (i.e., when the piston stroke is excessively returned beyond the expected amount thereof), there occurs an engagement delay to thereby cause a rotation rise of the engine due to the excessively small precharge oil pressure.